1. Field of the Invention
This invention relates to a plural-beam scanning optical apparatus, and particularly to a plural-beam scanning optical apparatus suitable for use in an apparatus such as a laser beam printer (L 13P) in which use is made of light source means having a plurality of light emitting portions and a plurality of light beams emitted from said light source means are directed to the surface of a photosensitive medium which is a scanned surface through a light deflector such as a rotatable polygonal mirror, whereby the surface of the photosensitive medium is scanned by the plurality of light beams at a time to effect, for example, the formation of image information.
2. Related Background Art
A so-called plural-beam scanning optical system designed to collectively scan the surface of a photosensitive medium which is a scanned surface by a plurality of light beams which can be optically modulated independently of one another has heretofore been proposed, for example, in Japanese Laid-Open Patent Application No. 54-158251.
Generally, to obtain a good quality of image with a high resolving power when a scanned surface is scanned by a plurality of light beams to thereby form an image, it is necessary to make the spot diameter of the light beams on the scanned surface small and densely form the spots of the light beams in the sub-scanning direction.
Generally, in a plural-beam scanning optical system, in order to make the spots in the sub-scanning direction dense, use is often made of light source means having a semiconductor laser array disposed obliquely with respect to the main scanning direction. Such a scanning apparatus is disclosed in U.S. Pat. No. 4,253,102.
FIG. 1 of the accompanying drawings is a schematic view of the essential portions of a plural-beam scanning optical apparatus according to the prior art.
In FIG. 1, reference numeral 11 designates light source means such as a semiconductor laser array. From a plurality of light emitting portions 11a and 11b provided on the light source means 11, central rays h.sub.a and h.sub.b are emitted in parallel to the optical axis g of a condensing lens 12. These central rays h.sub.a and h.sub.b pass through the focus F of the condensing lens 12, and pass through a cylindrical lens 13 having refractive power in the sub-scanning direction, and thereafter are incident on the deflecting surface 14a of a light deflector 14.
By this cylindrical lens 13, the light beams emitted from the light emitting portions 11a and 11b and passed through the condensing lens 12 are formed as linear images in the main scanning cross-section near the deflecting surface 14a. At this time, the central rays h.sub.a and h.sub.b from the light emitting portions 11a and 11b are incident on and reflected by the deflecting surface 14a at locations thereon spaced apart from each other in the direction of deflection of the light beams by the deflecting surface.
The light beams reflected by the deflecting surface 14a are imaged on the scanned surface 16 of a scanned medium such as a photosensitive member by a scanning lens system 15 comprising an anamorphic system.
FIG. 2 of the accompanying drawings is a schematic view showing the optical path in the sub-scanning cross-section from the light deflector 14 to the scanned surface 16. As indicated by solid lines in FIG. 2, the scanning lens system 15 is designed such that the deflecting surface 14a and the scanned surface 16 are in substantially conjugate relationship with each other in the sub-scanning cross-section.
In the plural-beam scanning optical apparatus shown in FIG. 1, the central rays h.sub.a and h.sub.b emitted from the light emitting portions 11a and 11b, respectively, of the light source means 11 are incident on the deflecting surface 14a at locations thereon spaced apart from each other, as previously described.
That is, of the center positions of the plurality of linear images on the deflecting surface 14a formed by the cylindrical lens 13, the center position of only the linear image corresponding to one of the plurality of light emitting portions can be installed at a desired location, for example, on the deflecting surface. However, the center position of the other linear image becomes spaced apart from the desired location.
That is, like the optical path indicated by broken lines in FIG. 2, the other linear image is formed at a location 14b deviating from the deflecting surface 14a. The center position Pa of the deviating linear image at this time is not imaged on the scanned surface 16, but is imaged at a point Pb spaced apart from the scanned surface 16, by the scanning lens system 15. That is, the deviating linear image becomes defocused on the scanned surface 16. The defocus amount at this time becomes greater as the light deflector 14 is rotated.
That is, there arises a problem that the spot diameter of the light beams on the scanned surface 16 increases and highly accurate optical scanning becomes impossible.
Also, as previously described, the light beams emitted from the light emitting portions 11a and 11b of the light source means 11 are incident on the deflecting surface 14a at locations thereon spaced apart from each other and are reflected and deflected, and this has led to a problem that as compared with a scanning optical apparatus using a single light beam, the deflecting surface must be made large and the deflector becomes correspondingly bulky and high-speed scanning at high accuracy becomes difficult.